1. Field of the Invention
This invention relates to a method of producing a seamless capsule, and more particularly to a method of producing a seamless capsule having a good shape and good preservability.
2. Related Art Statement
Conventionally, there has been widely known a method of producing a seamless capsule in which a concentric multi-layer solution is ejected from a multiple nozzle of a multiple orifice type, such as a double orifice and a triple orifice, into a hardning liquid to form a multiple-layer droplet and at least an outermost layer of the droplet is solidified to produce a seamless capsule.
Among the seamless capsules, one which is of simplest structure and has been widely used is a two-layer capsule having single core as the inner layer, which is made with a double orifice. In producing the two-layer capsule, in many cases, an outer layer is hardened by cooling.
Constitution of the single core two-layer capsule can be generally classified into two types, i.e., one containing an aqueous liquid as a material encapsulated therein and the other containing an oily liquid. As the outer layer, a lipophilic material such as wax is used in the former type, and a hydrophilic material such as gelatin, alginate or agar in the latter type.
For producing such a single core two-layer capsule, cooling type solidification is employed except for the case of chemical reaction type solidification where, for example, sodium alginate in the outer layer is solidified in reaction with an aqueous solution of a calcium salt. As a hardening liquid in the cooling type solidification, water has been used for the capsule having the lipophilic outer layer whereas an oily hardening liquid such as vegetable oil or liquid paraffin has been used for the hydrophilic outer layer.
Among the single core two-layer capsules, with regard to the capsule containing an oily material as the core, there have been proposed a large number of production methods and constitutions of the capsule, mainly in the field of an oral refrigerant. However, with regard to the capsule containing an aqueous solution therein, sufficient investigation has not yet been carried out.
For example, in U.S. Pat. No. 3,389,194, there is disclosed an example where water is used both as the core solution and as the hardening liquid, and wax is used as the outer layer. However, it only discloses a principle and does not describe the conditions required for respective components.
Also, in Swiss Patent No. 563,807, there is disclosed a similar method as in the above U.S. Patent, using water as the core solution, a melted paraffin as the outer layer and water as the hardening liquid. However, there is no further detailed description with respect to each component.
In recent years, as to constitution of a seamless capsule containing an aqueous solution, there have been developments to another direction. For example, in Japanese Unexamined Patent Laid-Open No. 56-89833, in order to contain an aqueous solution in the gelatin capsule, there is disclosed a method of protecting the gelatin outer layer by adding tannin to the gelatin when the aqueous solution is not acidic or adding an enteric coating agent when the aqueous solution is acidic, and using an oily substance such as vegetable oil as the hardening liquid.
Also, in Japanese Unexamined Patent Laid-Open No. 3-52639, there is disclosed a method of interposing a fatty acid ester of sucrose (sucrose acetate isobutyrate) between the aqueous core solution and the gelatin outer layer, and similarly using an oily substance as the hardening liquid.
As described above, whereas the method of using water as the core solution, an oily substance such as paraffin as the outer layer and water as the hardening liquid has been proposed for a long time, it has not been used practically.
This is because the method involves serious defects that uneven thickness of layers, i.e., eccentricity of layers or pin hole is likely to occur in the course of producing a seamless capsule so that the encapsulated water is lost by evaporation and thus, the quality of the capsule product is not preserved. It is considered that unlike a material such as gelatin which has flexibility and solidifies gradually, the outer layer comprising paraffin, etc. tends to produce cracks easily and rapidly solidifies at a solidifying temperature, and therefore the above-described defects are caused.
Thus, there is difficulty in the process of forming an outer layer with wax so that the methods of using gelatin as the outer layer have been proposed as mentioned above.
However, in the method disclosed in Japanese Unexamined Patent Laid-Open No. 56-89833, water sealing property of the outer layer is insufficient and loss of the encapsulated water is unavoidable. Also, in the method disclosed in Japanese Unexamined Patent Laid-Open No. 3-52639, fatty acid ester of sucrose used as the intermediate layer has low melting point and the ester becomes liquid at the room temperature so that the loss of water in the core solution can not be prevented, making the method impractical.
Thus, these methods of improving the gelatin outer layer can not be applied to except for a core solution such as containing a large amount of glycerin or polyethylene glycol in addition to water. It is the present status that there has not yet been found a method of producing a seamless capsule encapsulating a solution which mainly comprises water and having preservability for a long period of time.